Modern displays such as LCD and LED displays use red (R), green (G) and blue (B) light emitters to generate a color display image. Certain types of displays, including large displays, use R, G, and B light emitters (e.g., diodes) packaged together to define a full-color pixel, with an array of such pixels defining the display. The R, G, and B light emitters are referred to as “sub-pixels” since they collectively define the full-color (“color”) pixel, typically referred to as an “RGB” pixel.
Displays that use RGB pixels employ the RGB color model wherein the R, G, and B colors are added in different amounts to produce a wide variety of colors. The specific R, G, and B wavelengths used define a color gamut, which is a representation of the color space that can be produced by the light emitters. The color gamut is used to perform color management for the display.
While R, G, and B light can be used to generate a wide range of colors, in practice it is not the most efficient arrangement for all applications. It turns out that the addition of a white (W) sub-pixel to the R, G, and B sub-pixels to form an RGBW pixel has some advantages. For example, the W sub-pixel can be used to provide enhanced brightness, which allows for reducing the usage of the R, G, and B sub-pixels, which in turn increases the lifetime of the pixels and thus the display. Further, since it takes a certain amount of R, G, and B light to generate white light, it is more efficient to generate the needed amount of white light using only the W sub-pixel. In addition, it also enables the use of perceived picture quality enhancements. This can be, e.g., under the form of what is called “white boost,” where extra white is added to RGB to add extra gamma. This mimics the behavior of traditional CRT displays.
Unfortunately, RGBW pixels have some drawbacks, particularly for large-format displays that have dimensions measured in feet. One drawback is that the size of the white sub-pixel needs to be about the same as that for the R, G, and B sub-pixels so that RGBW pixel size is relatively large. Another drawback is that the conventional assumption that a white LED light emitter has greater brightness stability as compared to individual RGB LEDs is actually incorrect and that the color stability of a white LED can be problematic. The human eye is much more perceptive to color differences compared to brightness differences. Consequently, using a white sub-pixel to enhance brightness can exacerbate detrimental color stability issues.
Another drawback relates to displaying video information on the RGBW-pixel display and the amount of signal processing involved. Modern television and video cameras convert optical information into electrical signals. The electronic signals are then digitized, processed and packetized. The digitizing step results in a sharp image that is impervious to noise and other issues. For a display with RGBW pixels, substantial video processing is required for transmission of the digital signal over a digital interface, such as an SDI (serial digital interface), to the RGBW-pixel display. The SDI standard defines a coax cable connection for uncompressed video and also defines the video format in which data is sent in digital format of either 8 or 10 bit words to describe the digital image. Different resolution images run at different data rates but the concept of the video image is the same across the different speeds. Words are sent in a sequential fashion across the interface until an entire image has been sent, creating the image in a classic scanning fashion. The information sent across the SDI interface on a per pixel basis utilizes a luminance and color difference approach embodied in a chroma sub-sampling format. In particular, three words, Y, Cb and Cr, are sent per pixel, wherein Y=Luminance, Cb=Blue Color Difference=B−Y, and Cr=Red Color Difference=R−Y. The various chroma sub-sampling formats are expressed as Y:Cb:Cr, e.g., 4:2:2, and emphasize the transmission of brightness information over color information.
The receiving equipment accepts the stream of information coming from the SDI interface and is required to process the information and modify the data to be able to create a visual display. As most displays have RGB pixels to create an image, the display (video) processor has to take the information sent and turn it into RGB information. Thus, for each RGBW pixel, the amount of white light for the W sub-pixel is based on a calculation of the amount of light emitted by the R, G, and B sub-pixels per the three words Y, Cb, and Cr. An example of this type of calculation is described in the article by Candice H. Brown Elliott et al., entitled “Adding a White Subpixel,” Information Display, May 2005, pp. 26-31.